(1) Field of the Invention
The present invention relates to a printed wiring board and a process for manufacturing a printed wiring board, in particular, a printed wiring board including: a conductive wiring, which is formed on a surface of a board and has a solder land to which components to be mounted are electrically connected by solder; and a first electrically insulating layer formed on the conductive wiring, and a process for manufacturing such a printed wiring board.
(2) Description of the Related Art
As an example of a conventional printed wiring board, for example, a printed wiring board shown in FIG. 5 has been proposed. As shown in FIG. 5, the printed wiring board includes a board 10 and a conductive wiring 11 formed on a surface of the board 10. The conductive wiring 11 includes solder lands 16, to which electric components 12, 13, 14 are electrically connected by solder 15.
A resist film 17 as the first insulating layer is formed on a part of the conductive wiring 11, wherein the solder lands 16 is not formed on said part of the conductive wiring 11. There is a distance L between the solder land 16 of the electric component 12 and that of the electric component 13 so that the electric components 12 and 13 are electrically insulated from each other.
However, the distance L must be a certain length so as to secure electrical insulation, which causes a problem that a size of the printed wiring board cannot be decreased.
In this connection, flux is used in the soldering process for the purposed of preventing re-oxidation during the soldering and of decreasing surface tension of molten solder. In this case, there might be a problem that the flux remains between the conductive wiring 11 to be connected to the electric component 12 and the conductive wiring 11 to be connected to the electric component 13, that is, a flux residue 18 takes place.
Due to the flux residue 18, even if the distance L is provided upon designing so as to assure the electrical insulation between the electric components 12 and 13, there might be a problem that such insulation between the electric components 12 and 13 is deteriorated due to influence of high temperature-high humidity or humidity upon applying high voltage.
For solving the above problem, as shown in FIG. 6, there is an idea that a hole 19 is provided in a board 10 situated between the conductive wiring 11 to be connected to the electric component 12 and the conductive wiring 11 to be connected to the electric component 13. However, in this case, it is not possible to provide the electric component 14 on the rear side of the electric components 12 and 13 because of the hole 19, causing a problem that a high-density mounting cannot be realized.
As shown in FIG. 7, there is an idea that a silk 20 is provided on a board 10 situated between the conductive wiring 11 to be connected to the electric component 12 and the conductive wiring 11 to be connected to the electric component 13. However, there is a possibility that the flux residue 18 takes place on the silk 20, causing a problem.
As shown in FIG. 8A, there is an idea that the resist film 17 is formed to cover the periphery of the solder land 16 (Japanese Patent Application Laid-Open No. H11-251728). In this case, since there is a step between the solder land 16 and the resist film 17, therefore the flux never climbs over the resist film 17 to spread. Moreover, the insulation distance between the solder lands 16 becomes longer due to the step. Therefore, the distance L between the electric components 12 and 13 can be small owing to the step, so that a size of the printed wiring board can be decreased.
However, since there is the step between the solder land 16 and the board 10, the resist film 17 provided on the periphery of the solder land 16 is thinner than the resist film 17 located on the board 10. Further, since there is a possibility that a pin-hole takes place on the resist film 17, a short insulation distance cannot be sufficiently attained even if the step is provided. Moreover, the flux might climb over the resist film 17.
For solving these problems, as shown in FIG. 8B, it has been proposed that a thick resist film 17 is provided. However, it is very difficult to form a thick resist film 17 with a general method and there is a problem in terms of a cost.
As described in Japanese Utility Model Application Laid-Open No. 6-44177, Japanese Patent Application Laid-Open No. H10-270812 and Japanese Patent Application Laid-Open No. 2000-346421, so far there have been various measures to attain the electrical insulation between the conductive wirings 11. However, there has been no proposal as to a method of attaining electrical insulation between the solder lands 16.